The invention relates to a screen printing machine having a printing table and a squeegee which is guided reciprocatingly over the printing table in a frame, as well as a screen frame which can be swivelled around a shaft extending perpendicularly with respect to the movement of the squeegee, between the squeegee and the printing table.
In all known screen printing machines, the screen frame, behind the squeegee moving over the screen, must be raised such that the screen detaches from the dye applied to the sheet to be printed. It is disadvantageous that this lifting-off movement can be carried out at only one side of the screen frame. In the case of some printing operations, particularly when printing electric circuit boards, it is often not possible to place the so-called solder resist in the very narrow spaces between adjacent tracks, by means of a squeegee movement in only one direction. These narrow spaces are located on the printed circuit board in raised section resulting from removal of a copper layer by caustics. The spaces existing between the tracks are partly so narrow that they are not filled sufficiently when a squeegee movement takes place in only one direction. A squeegee movement in both directions, however, which in principle is also possible in the case of the known screen printing machines, can be accomplished in only one direction with a perfect lifting-off of the screen from the printed surface.
In this regard, it should be pointed out that, in screen printing, the distance at which the screen is located above the printing table and over which it is pressed downward onto the surface to be printed by the pressure of the squeegee always results in a so-called geometrical printing error which can be compensated at least partially by the one-sided raising of the screen frame behind the passing-through squeegee. It has also been suggested in German Patent (DE-PS No. 27 43 234) to swivel the screen frame upward, during the squeegee movement, not around a swivel shaft located at the end of the squeegee movement directly at the screen frame, but around a shaft located above the screen frame plane and at a distance from the end of the screen frame. Although, by means of this measure, it is possible to partially compensate the geometrically caused printing error and possibly also the printing error caused by the expansion of the network, as in all other known screen printing machines, this is possible only if the squeegee is moved over the screen in one direction.
An object of the invention is therefore to develop a screen printing machine of the initially mentioned type such that it is possible to raise the screen frame for both moving direction so that, in both moving directions, at least partially, a compensation is possible of the printing error occurring during screen printing.
For achieving this object, it is suggested, in the case of a screen printing machine of the initially mentioned type, to place the swivel shaft through the center plane of the screen frame, which extends perpendicularly with respect to the movement of the squeegee, and to provide devices for the raising of the screen frame on both sides of the swivel shaft. As a result of these measures, the screen frame, as a function of the movement of the squeegee, can always be raised such that it is lifted up behind the continuing squeegee. Thus it becomes possible to compensate a printing error even if, for example, during the production of printed circuit boards, printing must take place by means of a double movement of the squeegee in both directions.
This new development also offers the advantage that, as a result of the swivelling movement of the screen frame around its center, the part of the screen frame which is located in front of or behind the swivel shaft--viewed during the movement of the squeegee--carries out different relative motions with respect to the printing table which are opposed to one another. These different relative movements, which result in an opposite shifting of parts of the screen with respect to the printing table, permit, in a particularly simple manner, that the geometrical printing error is compensated which has different preceding signs also over the length of the screen frame. However, at the same time, as a result of the screen printing process, a certain compensation of the length expansion of the screen may be achieved in that the swivel movement of the screen frame does not begin before the movement of the squeegee starts, and therefore larger swivelling angles are not reached before the squeegee has covered a larger distance on the screen. Therefore, by means of the new development, analogously to the increasing screen expansion during the movement of the squeegee, an opposed compensation may be made possible.
In a particularly simple manner, the new device may be implemented in that the swivel shaft is formed by two shaft journals which each project from the screen frame toward the outside and which are each disposed in a link which can be displaced perpendicularly with respect to the printing table. In this case, the link may be laterally guided in a vertical guide rail which is fixedly arranged at the frame. In order to achieve a low-friction guiding, it is advantageous to equip the link with two rollers which each rest laterally against the guide rail. This construction makes it possible to maintain the distance of the screen frame with respect to the printing table in the end area of the movement of the squeegee despite the swivelling movement around the center line. However, the displacement of the screen frame with respect to the printing table according to the invention takes place nevertheless.
With respect to being able to raise the screen frame selectively at both ends, a simple solution can be achieved by providing carriages which each can be displaced in parallel with respect to the longitudinal edge of the screen frame, a lifting slope being assigned to each of them, for controlling movement of lateral guide pins of the screen frame. By means of a relative displacement of the carriages with respect to the screen frame, the guide pins run up against the lifting slopes, and as a result, the screen frame is raised.
In order to, as indicated previously, achieve at the same time that during the lifting movement, the distance of the screen frame with respect to the table surface does not change, it is very advantageous for the lifting slopes to be part of a slot guide in the carriages, which slot guide consists of two slot portions enclosing an obtuse angle, the lengths of these slot portions being the same as measured in a direction parallel to the printing table. In this case, one slot portion may extend in parallel with respect to the printing table so that, during a relative displacement of the carriages, an end area--specifically the end area containing the lateral guide pins--continues to have the same distance from the printing table, and only the remaining screen frame is raised. Expediently, the construction is such in this case that the guide pins, which engage in the slot portion, which extends in parallel with respect to the printing table, are also arranged in the area of the ends of the printing table. Correspondingly, carriages with slot guides are assigned to each of the two ends of the screen frame on both sides, and these slot guides in the carriages assigned to the opposite ends of the screen frame are constructed in mirror symmetry with respect to the center plane of the screen frame. When one end of the screen frame is raised, the other end is guided in parallel with respect to the printing table and vice versa.
For the relative displacement of the carriages with respect to the printing table and the screen frame, it is advantageous and simple to provide a displacing drive for both carriages, which consists of connecting rods connecting both carriages with a pivoted lever, which is pivoted at the frame of the screen printing machine and which can be caused to carry out its reciprocating movement in a controlled manner by means of a drive.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.